This invention relates to a surface having a tunable electromagnetic impedance, and includes a conductive sheet of metal or other conductor, covered with an array of resonant elements, which determine the surface impedance as a function of resonance frequency. The surface impedance governs the reflection phase of the conductive sheet. Each resonant element is individually tunable by adjusting a variable capacitor, thereby controlling the electromagnetic impedance of the surface. By having a tunable, position-dependent impedance, this surface can be used to focus a reflected Radio Frequency (RF) beam by forming an effective Fresnel or parabolic reflector or to steer a reflected wave by forming an effective prism or grating. The tunable impedance surface can be used to steer or focus an RF beam, which is important in such fields as satellite communications, radar, and the like.
Prior art approaches for RF beam steering generally involve using phase shifters or mechanical gimbals. With the tunable surface disclosed herein, beam steering is accomplished by variable capacitors, thus eliminating expensive phase shifters and unreliable mechanical gimbals. The variable capacitors can be controlled electronically using variable dielectrics, or tuned using devices to impart relatively small mechanical motion such as microelectromechanical (MEM) switches.
Focusing an RF beam by a flat surface has been accomplished in the prior art by using an array of nearly resonant half-wave dipoles, which are designed to have a particular reflection phase. However, if such a structure is to include a ground plane, this prior art structure must be one-quarter wavelength thick. In the present invention, the thickness of the tunable surface is much less than one-quarter wavelength. The available bandwidth is partly determined by the tunability of the small resonant elements on the surface, which are tuned by variable capacitors.
The present application is related to U.S. patent application Ser. No. 09/537,921 entitled xe2x80x9cAn End-Fire Antenna or Array on Surface with Tunable Impedancexe2x80x9d filed Mar. 29, 2000 and to U.S. patent application Ser. No. 09/537,722 entitled xe2x80x9cAn Electronically Tunable Reflectorxe2x80x9d filed Mar. 29, 2000 the disclosures of which are hereby incorporated herein by this reference.
The prior art includes U.S. Pat. No. 4,905,014 to Daniel G. Gonzalez, Gerald E. Pollen, and Joel F. Walker, xe2x80x9cMicrowave phasing structure for electromagnetically emulating reflective surfaces and focusing elements of selected geometry.xe2x80x9d This patent describes placing antenna elements above a planar metallic reflector for phasing a reflected wave into a desired beam shape and location. It is a flat array that emulates differently shaped reflective surfaces (such as a dish antenna).
The prior art includes U.S. Pat. No. 5,541,614 to Juan F. Lam, Gregory L. Tangonan, and Richard L. Abrams, xe2x80x9cSmart antenna system using microelectromechanically tunable dipole antennas and photonic bandgap materialsxe2x80x9d. This patent shows how to use RF MEMS switches and photonic bandgap surfaces for reconfigurable dipoles.
The prior art includes RF MEMS tunable dipoles xc2xc wavelength above a metallic ground plane, but this approach results in limited bandwidth and limited tunability. We improve on this approach by replacing the reconfigurable dipole array with a tunable impedance surface, resulting in a thinner structure, with broader bandwidth.
The prior art further includes a pending applications of D. Sievenpiper, E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d, U.S. provisional patent application, Ser. No. 60/079,953, filed on Mar. 30, 1998.
A conventional high-impedance surface, shown in FIG. 1, consists of an array of metal top plates or elements 10 on a flat metal sheet 12. It can be fabricated using printed circuit board technology with the metal plates or elements 10 formed on a top or first surface of a printed circuit board and a solid conducting ground or back plane 12 formed on a bottom or second surface of the printed circuit board. Vertical connections are formed as metal plated vias 14 in the printed circuit board, which connect the elements 10 with the underlying ground plane 12. The metal members, comprising the top plates 10 and the vias 14, are arranged in a two-dimensional lattice of cells, and can be visualized as mushroom-shaped or thumbtack-shaped members protruding from the flat metal surface 12. The thickness of the structure, which is controlled by the thickness of the printed circuit board, is much less than one wavelength for the frequencies of interest. The sizes of the elements 10 are also kept less than one wavelength for the frequencies of interest. The printed circuit board is not shown for ease of illustration.
Turning to FIG. 2, the properties of this surface can be explained using an effective circuit model or cell which is assigned a surface impedance equal to that of a parallel resonant LC circuit. The use of lumped cells to describe electromagnetic structures is valid when the wavelength is much longer than the size of the individual features, as is the case here. When an electromagnetic wave interacts with the surface of FIG. 1, it causes charges to build up on the ends of the top metal plates 10. This process can be described as governed by an effective capacitance C. As the charges slosh back and forth, in response to a radio-frequency field, they flow around a long path P through the vias 14 and the bottom metal surface 12. Associated with these currents is a magnetic field, and thus an inductance L. The capacitance C is controlled by the proximity of the adjacent metal plates 10 while the inductance L is controlled by the thickness of the structure.
The structure is inductive below the resonance and capacitive above resonance. Near the resonance frequency,       ω    =          1              LC              ,
the structure exhibits high electromagnetic surface impedance.
The tangential electric field at the surface is finite, while the tangential magnetic field is zero. Thus, electromagnetic waves are reflected without the phase reversal that occurs on a flat metal sheet. In general, the reflection phase can be 0, xcfx80, or anything in between, depending on the relationship between the test frequency and the resonance frequency of the structure. The reflection phase as a function of frequency, calculated using the effective medium model, is shown in FIG. 3. Far below resonance, it behaves like an ordinary metal surface, and reflects with a xcfx80 phase shift. Near resonance, where the surface impedance is high, the reflection phase crosses through zero. At higher frequencies, the phase approaches xe2x88x92xcfx80. The calculated model of FIG. 3 is supported by the measured reflection phase, shown for an example structure in FIG. 4.
A large number of structures of the type shown in FIG. 1 have been fabricated with a wide range of resonance frequencies, including various geometries and substrate materials. Some of the structure were designed with overlapping capacitor plates, to increase the capacitance and lower the frequency. The measured and calculated resonance frequencies for twenty three structures with various capacitance values are compared in FIG. 5. Clearly, the resonance frequency is a predictable function of the capacitance. The dotted line in FIG. 5 has a slope of unity, and indicates perfect agreement. The bars indicate the instantaneous bandwidth of the surface, defined by the frequencies where the phase is between xcfx80/2 and xe2x88x92xcfx80/2.
Features of the present invention include:
1. A device with tunable surface impedance;
2. A method for focusing an electromagnetic wave using the tunable surface; and
3. A method for steering an electromagnetic wave using the tunable surface.
This invention provides a reconfigurable electromagnetic surface which is capable of performing a variety of functions, such as focusing or steering a beam. It improves upon the high-impedance surface, which is the subject of U.S. Provisional Patent Ser. No. 60/079,953, to include the important aspect of tunability, as well as several applications. The tunable structure can have any desired impedance, and thus any desired reflection phase. Therefore, by programming the surface impedance as a function of position, it can mimic such devices as a Fresnel reflector or a grating, and these properties can be reprogrammed electronically.
The present invention provides, in one aspect, a tuneable impedance surface for steering and/or focusing a radio frequency beam, the tunable surface comprising: a ground plane; a plurality of top plates disposed a distance from the ground plane, the distance being less than a wavelength of the radio frequency beam; and a capacitor arrangement for controllably varying the capacitance of adjacent top plates.